Acoustic measurement systems used in logging while drilling (“LWD”), as well as wireline logging applications, have been found to be effective in obtaining various formation property data. During operation, acoustic waves are fired from the transmitters located on the tool. The acoustic responses are then captured by an array of receivers spaced apart along the tool body. The recorded acoustic responses will be processed to determine the compressional and shear slowness of the formation.
In slow formations, where the shear wave velocity is slower than the sound velocity in the mud, there is no critical shear wave from the formation. To resolve this issue, specially excited borehole waves, such as dipole or quadrupole modes, need to be used. These working modes will asymptote to formation shear at low frequency. However, in these modes, the energy of the transferred acoustic wave reduces by one order compared to the monopole. Therefore, the transmitter must provide enough energy for the waves to propagate through the formation. One approach is to increase the strength of the acoustic transmitter or the sensitivity of the receivers. Although this approach is effective, this will increase the tool mode as well as the noise in the recorded data logs. Moreover, this increase in the tool mode and noise is much more challenging in LWD where the tool body is thick and rigid.